Which AWS tool gives users the ability to plan their service usage, service costs, and instance reservations, and also allows them to set custom alerts when their costs or usage exceed established thresholds?

Explanations:

Cost Explorer allows users to view and analyze their spending but does not offer budgeting or alerting features.

AWS Budgets enables users to set custom cost and usage budgets and alerts when thresholds are exceeded, making it the best option.

AWS Cost and Usage Report provides detailed billing data but does not allow for budgeting or alerting functionality.

Reserved Instance reporting focuses on the usage of reserved instances and does not provide budgeting or alert capabilities.